The proliferation of comparatively high-resolution digital imaging devices, such as digital still cameras, has led to the pursuit of increasingly higher-resolution photo manipulation, printing and other tools. However, in order to contain cost, many consumer-grade digital color cameras are single-sensor digital cameras. As the name implies, in a single-sensor digital camera only a single image sensor is used to capture color information for each pixel in a color image. Each image sensor, which is typically a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS), is part of a sensor array that together represent the pixels of a color image. Each image sensor can only generate information about a single color at a given pixel. These single color pixels are used to comprise an image in a so-called “Raw” format. The expanding digital image market has brought recognition that the raw image files generated by digital cameras and other devices represent an opportunity to extract the highest possible level of detail from the device.
A color image, however, is represented by combining three separate monochromatic images. In order to display a color image, all of the red, blue and green (RGB) color values are needed at each pixel. In an ideal (and expensive) camera system, each pixel in the sensor array would be provided with three image sensors—each one measuring a red, green or blue pixel color. In a single-sensor digital camera, however, only a single red, blue or green color value can be determined at a given pixel. In order to obtain the other two missing colors, they must be estimated or interpolated from surrounding pixels in the image. These estimation and interpolation techniques are called “demosaicing” algorithms.
The term “demosaicing” is derived from the fact that a color filter array (CFA) is used in front of the image sensors, with the CFA being arranged in a mosaic pattern. This mosaic pattern has only one color value for each of the pixels in the image. In order to obtain the full-color image, the mosaic pattern must be demosaiced. Thus, demosaicing is the process of interpolating back the raw image captured with a mosaic-pattern CFA, so that a full RGB value can be associated with every pixel.
Today, raw sensor data is converted into RGB data in two ways. The data may be demosaiced by the hardware of an image capture device (e.g., cameras and viewers). Alternatively, the raw data may be demosaiced and processed by a personal computer (PC). For example, the data may be downloaded from a camera onto a PC where it may be processed by an application or an operating system to create an image stored in a more readily processed format, such as JPEG (Joint Photographic Experts Group) or TIFF (Tagged Image File Format). Compared to the Raw format, these more readily processed formats are inferior and lead to, for example, loss in color depth and poor compressions.
Demosaicing on an image capture device and on a PC differ in at least one significant way. On-device demosaicing often requires only a fraction of a second, while the same processing can take 30 seconds or more on a PC. With the premium modern computer users place on speed and performance, the PC's demosaicing delay is unacceptable to most users, and more readily processed formats such as JPEG are more commonly used. In short, the poor speed of performance experienced when working with raw image data causes users to select the more readily processed formats, despite the superior level of detail and precision offered by raw image data.